Generally, a comparator is a circuit that compares two voltages and outputs a signal indicating a result of the comparison. Comparators have many different applications. For instance, comparators are commonly utilized in pulse generators, square-wave generators, triangular-wave generators, pulse-width modulators, level detectors, zero-crossing detectors, pulse generators, line receivers, limit comparators, voltage-controlled oscillators, A/D converters, time delay generators, and other circuits.
One common circuit utilized to implement a comparator is a differential amplifier, which provides a high output when one of the two signals is higher and a low output when the other of the two signals is higher. One of the inputs to the differential amplifier is usually a reference voltage. The other input comprises a signal which is compared to the reference voltage, so as to obtain an indication as to which of the two inputs to the differential amplifier is larger in magnitude.
An important characteristic of a comparator is the response time or propagation delay introduced thereby. The response time can be defined as the time between the input voltage transition and some specified point on the output voltage transition. An accurately defined trip point and process dependent parameters of the comparator are also very important considerations in utilizing the comparator. Furthermore, it is sometimes desirable to compare a signal to more than one signal. It is certainly possible to construct a multi-input comparator circuit using a plurality of contemporary comparators or the like. This may be accomplished simply by using a first comparator to compare the two reference voltages to determine the higher voltage, then switching the highest reference voltage to the second comparator to compare the highest reference voltage to another signal. Alternatively, a signal can be compared to one reference voltage on a first comparator while simultaneously comparing the same signal to a second reference voltage on a second comparator, then applying logic gates to determine whether or not the signal is greater than both reference voltages.
However, to do so with contemporary devices inherently results in at least a doubling of the propagation delay. In some applications, such a propagation delay is not desirable. Furthermore, more sophisticated comparison of one input voltage to two or more other voltages cannot be accomplished using typical methods. For instance, comparing a voltage V2 with two other voltages V1 and V3, where the trip point of the comparator is a multiple of V1 summed with V3 is not possible using a basic combination of traditional comparators.
In view of the foregoing, it is desirable to provide a comparator for comparing a signal to one or more other voltages, where the comparator mitigates propagation delay, has a fast response time. Furthermore, it is desirable for providing a comparator that has a well defined trip point that is not process dependent.